A biochemical assay has been developed for the src protein of Harvey sarcoma virus. The p21 src protein has been found to have a high affinity for guanine nucleotides but not adenine nucleotides. This assay, based on the binding of guanine nucleotides, provides a critical first step in elucidating a biochemical pathway for malignant transformation. A system for studying the mechanisms involved in erythroleukemia has been developed. Three distinct viruses can induce erythroleukemia in this system. Each virus causes somewhat different form of erythroleukemia thus allowing for the first time the beginning of a biochemical mechanism. This system and subclassification should have the same fundamental significance for cancer diagnosis as the ability to distinguish vitamin B12 vs iron deficient anemia did for nonmalignant erythroid diseases some three to four decades ago.